1. Field of the Invention
This invention relates to implantable biomedical devices, and more particularly, relates to blood contacting sensing devices and a method for their protection against degradation induced by the blood.
2. Background of the Invention
In current diagnostic and therapeutic medical practice, many situations arise in which an instrument must be implanted and maintained in a patient's body for extended periods of time. For example, implanted sensors for gases, electrolytes and pH which depend on a component of a body fluid reaching the sensor for a chemical reaction are well-known.
In other cases, the sensor measures a physical effect, such as blood pressure, by converting a pressure flex on the sensor to an electrical charge. In these cases, it is often essential that the body fluid be prevented from contacting the sensing element, because body fluids in general contain ions in an aqueous environment, and ions and other constituents of body fluids are often deleterious to the sensing element particularly when an electrical charge is to be developed. In addition, water vapor passes easily through most materials and itself catalyses reactions that may corrode various device components.
Exemplary of devices adapted for implantation is that described by Wallace et al. in U.S. Pat. No. 4,785,822. The Wallace et al. device includes a pressure transducer covered by a cap for monitoring pressure in body compartments such as the uterus. The transducer and cap are disposed in a flexible boot, and aligned hole in the cap and boot are filled with a silicone gel. The gel serves as a hydraulic fluid which transmits external pressure in the body compartment to the transducer and also provides a water tight seal to prevent body fluid from reaching the transducer.
Hutchins et al., in U.S. Pat. No. 3,710,781, discloses a catheter tip pressure transducer for blood pressure measurement. The transducer is covered by a rubber sheath which provides a seal against entrance of the blood into the transducer compartment.
Protection of implants from damage by body fluids has been summarized by Troyk in Sensors Expo Proceedings, 1988, page 308A-1. Troyk discusses the state of the art in packaging for implantable sensors, and states that, while blood compatible chemical sensors have been developed, they cannot be used with implanted systems because of packaging difficulties.
Coatings have been extensively studied for the protection of implants. Matsuo et al., in Sensors and Activators, 9, 115 (1986) discloses a parylene-coated reference ion sensitive field effect transistor fabricated by cleaning of the silicon surface with an oxygen plasma and depositing a 100 nm parylene coating thereon.
Yasuda et al., in Biomedical Sciences Instrumentation, 17, 109 (1981), reports that vapor deposited parylene coating adhere well to polymeric surfaces but poorly to metal and glass, and that the poor adhesion to metal and glass may be overcome by glow discharge depositing a primer coating of polymer and vapor depositing a coating of parylene over the polymer. Similarly, Nichols et al., in Biomedical Sciences Instrumentation, 23, 57 (1987), states that no single off-the-shelf polymeric material has sufficient biocompatibility and adherence to various implant materials to provide insulation to a sensor exposed to the hostile ionic environment of extracellular fluid. Thus, Nichols et al. teaches a trilayer coating for sensor implants consisting of a first glow discharge polymerized and deposited layer of methane, a second layer of glow discharge polymerized and deposited parylene C thereon and an outside layer of biocompatible parylene C vapor deposited thereon.
Blood contacting medical implants face the additional problem of the thrombogenicity associated with most foreign materials in contact with the blood. Kanda et al., in Electronic Letters, 17, 558 (1981) discloses a study of clotting times of various surfaces in contact with blood.
Much effort has been expended by many workers in an effort to construct a direct blood contacting device which can be implanted in a patient's blood stream and provide continuous blood pressure monitoring for a protracted period of time without sustaining damage or inducing thrombosis. While the above disclosures have addressed the problem, to date, no such device exists. The present invention provides a solution to this problem.